The present invention is directed to a luminescent material doped with trivalent and/or divalent ions, and more particularly to a YMO4:Eu,L material doped with Tb3+ and/or Mg2+ and used as a lamp phosphor.
A luminescent material absorbs energy in one portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. A luminescent material in powder form is commonly called a phosphor, while a luminescent material in the form of a transparent solid body is commonly called a scintillator.
Most useful phosphors emit radiation in the visible portion of the spectrum in response to absorption of radiation which is outside the visible portion of the spectrum. Thus, the phosphor performs the function of converting electromagnetic radiation to which the human eye is not sensitive into electromagnetic radiation to which the human eye is sensitive. Most phosphors are responsive to more energetic portions of the electromagnetic spectrum than the visible portion of the spectrum. Thus, there are powder phosphors which are responsive to ultraviolet light (as in fluorescent lamps), electrons (as in cathode ray tubes) and x-rays (as in radiography).
Two broad classes of luminescent materials are recognized. These are self-activated luminescent materials and impurity-activated luminescent materials.
A self-activated luminescent material is one in which the pure crystalline host material, upon absorption of a high energy photon, elevates electrons to an excited state from which they return to a lower energy state by emitting a photon. Self-activated luminescent materials normally have a broad spectrum emission pattern because of the relatively wide range of energies which the electron may have in either the excited or the lower energy states. Thus, any given excited electron may emit a fairly wide range of energy during its transition from its excited to its lower energy state, depending on the particular energies it has before and after its emissive transition.
An impurity activated luminescent material is normally one in which a non-luminescent host material has been modified by inclusion of an activator species which is present in the host material in a relatively low concentration, such as in the range from about 200 parts per million to 1,000 parts per million. However, some phosphors require several mole or atomic percent of activator ions for optimized light output. With an impurity activated luminescent material, the activator ions may directly absorb the incident photons or the lattice may absorb the incident photons and transfer the absorbed photon energy to the activator ions.
The photon absorbed by the lattice may create mobile migrating electrons and holes in the lattice. Due to favorable charge configurations, the migrating electrons and holes are trapped at the activator ions, where they recombine and emit a photon of luminescent light.
Alternatively, if the photon is absorbed directly by the activator ion, the photon raises one or more electrons of the activator ions to a more excited state. These electrons, in returning to their less excited state, emit a photon of luminescent light.
In many commonly employed impurity activated luminescent materials, the electrons which emit the luminescent light are d or f shell electrons whose energy levels may be significantly affected or relatively unaffected, respectively, by the surrounding crystal field. In those situations where the activator ion is not significantly affected by the local crystal field, the emitted luminescent light is substantially characteristic of the activator ions rather than the host material and the luminescent spectrum comprises one or more relatively narrow emission peaks. This contrasts with a self-activated luminescent material""s much broader emission spectrum.
When a host lattice absorbs the incident photon (i.e. the excitation energy) and transfers it to the activator ion, the host lattice acts as a sensitizer. The host lattice may also be doped with a sensitizer atoms. The sensitizer atoms absorb the incident photon either directly, or from the host lattice, and transfer it to the activator ion. For example, in a YVO4:Eu3+ phosphor, incident ultraviolet radiation excites the vanadate group (VO43xe2x88x92) of the host YVO4 lattice, which transfers the excitation energy to the Eu3+ activator ions present on the Y cationic sites in the lattice. The emission spectra of Eu3+ activator ions in the YVO4:Eu3+, phosphor comprises sharp lines in the red spectral range. These lines correspond to transitions from the excited 5D0 level to the 7F2 levels of the 4f6 configuration.
The YVO4:Eu3+, phosphor is commonly used in a high pressure mercury vapor lamp (HPMV). This lamp typically has a relatively low emission in the red spectral region. When the HPMV lamp is used in outdoor applications, red objects appear dull brown. This leads to practical disadvantages. For example, it becomes hard to distinguish red objects from brown objects in an area lighted by the HPMV lamp.
Therefore, the inside surface of the HPMV lamp bulb is coated with the YVO4:Eu3+ phosphor to correct for the lamp""s lack of emission in the red spectral range. It is thought that the phosphor absorbs the long and short UV radiation from the lamp and emits radiation in the red spectral range (i.e. visible light). Since HPMV lamps reach operating temperatures of 200-2500xc2x0 C. and have an operating rated life of about 24,000 hours, it is important that the color correcting phosphor be resistant to radiation damage and exhibit a high lumen maintenance. Radiation damage is the characteristic of a luminescent material in which the quantity of light emitted by the luminescent material in response to a given intensity of stimulating radiation decreases after the material has been exposed to a high radiation dose. Lumen maintenance is the ability of a luminescent material to resist radiation damage over time. Luminescent materials with a high resistance to radiation damage over time have a high lumen maintenance.
However, the YVO4:Eu3+ phosphor may exhibit an appreciable decrease in light output after several hundred hours of operation of the HPMV lamp at temperatures between 200-250xc2x0 C. Therefore, the YVO4:Eu3+ phosphor may have a lumen maintenance which is undesirably low in some cases.
In view of the foregoing, it would be desirable to provide a red emitting phosphor material that exhibits an adequate lumen maintenance. It would also be desirable to provide a method of making such a phosphor.
The present invention provides a composition of matter, comprising YMO4:Eu,L where M comprises at least one of vanadium and phosphorus and L comprises at least one of a trivalent rare earth ion excluding Eu and a divalent metal ion.
The present invention also provides a lamp, comprising a bulb having inside and outside surfaces, a gas discharge envelope, a base, at least two gas discharge electrodes and a phosphor on at least one of the surfaces of the bulb, wherein the phosphor comprises YMO4:Eu,L where M comprises at least one of vanadium and phosphorus and L comprises at least one of a trivalent rare earth ion excluding Eu and a divalent metal ion.
Furthermore, the present invention provides a method of making a composition of matter by mixing carbonate, oxide, oxalate or nitrate compounds of yttrium, vanadium, europium and at least one of a trivalent rare earth element excluding europium and a divalent metal element and heating a resulting mixture.